A rotary wing aircraft provides a complex vibratory environment. Vibrations emanate from many different sources including the main rotor, the tail rotor, gearboxes, linkages and engines. Vibrations will vary in intensity and frequency depending on the speeds and relative speeds of rotation of the main and tail rotors, load factors, structural deformation, resonances inherent in the airframe, and aerodynamic forces.
Vibrations cause premature failure of mechanical components such as gears and bearings, damage to avionics, flight instruments, fatigue to the airframe and discomfort to passengers and aircrew. Some underlying cause of vibrations include imbalances in rotating parts, uneven friction, meshing of gear teeth, parts that are dragging together, etc. Traditionally, helicopter vibrations have been managed or suppressed by balancing and alignment of rotating parts, reduction of friction, the use of vibration isolation mounts, installation of damping structures, absorption materials, and the like.
More recently, electronic systems have been devised to monitor and manage vibrations on rotary wing aircraft. Rotor track and balance systems such as the Rotor Analysis and Diagnostic System (RADS) have focused on providing information that can be used in flight to adjust pitch links, blade weights and trim tabs for smoother operation. Still other systems such as the Active Control of Structural Response system (ACSR) made by Agusta-Westland have been designed to reduce vibrations of the main rotor by active control systems that employ high-frequency force-actuation within the helicopter's structure. See, for example, U.S. Pat. No. 5,853,144.
Unfortunately, not all harmful vibrations have been mitigated by RADS, ACSR, or similar systems. One such vibration that is not mitigated by an ACSR or RADS type systems is referred to a limit cycle oscillation (LCO). In general, an LCO is defined as an oscillation of finite duration and finite amplitude which will return to a steady state value without additional external influences placed upon the system other than those found in the normal system environment. In some helicopters, such as the EH-101/AW-101, manufactured by Agusta-Westland, LCO's of a significant amplitude and duration have been detected in the vicinity of the tail rotor. These LCO events are unpredictable, occur only rarely and emanate far enough away from the cockpit that they have not been perceived by members of the aircrew until they have reached a magnitude that could cause damage to the aircraft.
In an effort to better understand LCO's in the EH-101/AW-101 and similar helicopters, vibration sensors have been placed in the vicinity of the tail rotor and the signals monitored either on the ground, via telemetry, or by an operator while in the air. Because LCO events have been extremely rare and have taken place under a variety of seemingly unrelated conditions, their causes are not yet well understood. However rare they may be, the consequences of LCOs are potentially catastrophic. Thus, the recommended course of action when a significant LCO is detected is to land the aircraft as soon as practicable, before damage to the airframe can occur. Tasking an aircrew member to monitor for LCO events is simply not practical. Thus, there is an immediate need for a system to detect and alert a helicopter aircrew to unsafe vibration levels from LCOs in the AH-101/AW-101 and similar rotary wing aircraft, before damage from an LCO can take place.